Toremifene, the Z isomer of 4-chloro-1,2-diphenyl-1-[4-[2-(,N-dimethyl-amino)ethoxy]phenyl]-1-butene, is a triphenylethylene derivative of formula

Toremifene has antiestrogenic activity and is useful in the treatment of hormone dependent breast cancer. Only the Z isomer has the useful antiestrogenic activity, the E isomer being estrogenic. Thus, the isomeric purity of toremifene is critical, since the presence of E-isomer may counteract the antiestrogenic effect of toremifene. A method for preparing toremifene, its analogs and salts has been described in U.S. Pat. No. 4,696,949. As described therein, toremifene can be conveniently prepared in a single step reaction by treating corresponding triphenyl diol compound with thionyl chloride, wherein a mixture of E and Z isomers is obtained. U.S. Pat. No. 4,696,949 describes separation of the E or Z isomers of toremifene analogs or their salts by crystallization from solvents such as hexane-ethanol (95:5), toluene-petrol ether (1:1) and toluene. However, it has been found that these methods have drawbacks related to the yield, purity or color of the end product. Thus, there is still a need for an improved method for isolating of substantially pure Z isomer of toremifene.